The present invention relates to the field of synthetic aperture radar (SAR), and in particular to a signal processing apparatus for a SAR system having a rotating antenna.
German Patent document DE-PS 39 22 086 and the dissertation by Dr.-Ing. Helmut Klausing, entitled "Implementability of a Radar Device Comprising a Synthetic Aperture by Means of Rotating Antennas", MBB-Publication, MBB-UA-1150-89-Pub=OTN-029299, 1989, disclose a radar device which has a transmitter and a receiver as well as an antenna for the transmitting and receiving of radar pulses. The antenna is arranged on the end of a rotating arm, for example, a rotating arm of a helicopter rotor or of a turnstile located above the rotor axis. A radar device of this type comprising a synthetic aperture on the basis of rotating antennas is called a ROSAR-device. The received signals are demodulated and are stored intermediately, and are then correlated with reference functions. These reference functions are calculated and preset based on the illumination geometry of the radar device.
The parameters for calculation and presetting of the reference functions are the distance intervals to be measured, the transmission frequency, the length of the rotating arm, the angle of rotation range of the antenna from which the signals are received back, the number of transmitted pulses, as well as the height of the rotating antenna above the ground. The correlation result will be appropriately displayed, for example, on a monitor.
A radar device of this type may be used in approximate real time in the on-line operation and, thus, can be used, for example, not only in the field of cartography, in obstacle warning operations or as a landing aid, but also for the purpose of target reconnaissance and target tracking. The processor of this known ROSAR-device has several modules in order to subdivide the multiple and complex computing tasks and therefore permit an on-line operation.
In this known apparatus, the result for each distance interval is always obtained by the correlation of the received signal with a reference function that is valid for this distance interval.
In German Patent document DE-PS 39 22 086, the problem of the resolution of a ROSAR-device is addressed only briefly, specifically only the so-called range curvature problem. The resolution of a ROSAR-device in the lateral and the radial direction is determined by parameters which are partially coupled with one another. These parameters include the wavelength .gamma. of the transmitted signal and the length L of the rotating antenna arm; the apex angle of the antenna .gamma.; the distance R.sub.GO between the antenna and the center line of the illuminated strip; the height H.sub.O of the antenna above the ground; the pulse repeating frequency .function..sub.p and the duration of the transmitted pulses, and therefore the number Z.sub.s of the pulses for each aperture length S; the duration t.sub.e of the received echo signals; and the scanning rate for the distance intervals, etc.
The lack of resolution of a ROSAR-device is caused by the changing distance to the scanned object during the integration period of the received signal. This has the result that the resolution cells or storage cells which are assigned to the individual distance intervals are curved. If the path difference .DELTA.R.sub.s in the radial direction caused by this curvature is equal to or larger than the dimension of the resolution cell (also in the radial direction .DELTA.R.sub.Smin =(c.multidot..tau.)/2) a point target is imaged in two adjacent or even in several adjacent resolution cells. That is, the point target is imaged in a blurred manner. The signal course of the received signal in the azimuth will then seem to be bent in a curve-type manner. With respect to the deviations, reference is made to the above-mentioned MBB-Publication, Pages 50 to 54.
There is therefore needed a ROSAR-device of the above-described type that minimizes the imaging defects.
This need is met according to the present invention, by a radar device comprising at least one transmitter and one receiver. The radar device has at least one antenna arranged for the transmitting and receiving of radar pulses on the end of a rotating arm. The radar device further comprises a device for demodulating and intermediately storing the received signals, including devices for forming and storing reference functions as a function of the illumination geometry of the radar device, the distance intervals to be measured, the angle of rotation ranges, the transmitted pulses as well as the height of the rotating antenna above ground. A processor circuit is provided for subdividing the distance range illuminated by the antenna into individual distance intervals, and for determining the reference functions in these distance intervals. A correlator correlates the received signals to the reference functions. A display device is provided for displaying the correlation result. The processor circuit comprises a comparator circuit for estimating the distortion of the stored distance intervals caused by the movement of the antenna. This circuit acts upon a further circuit for correcting the range curvature deviation.
Accordingly, the processor circuit is expanded by a comparator module in which an imaging defect is recognized. The imaging defect is caused because of the range curvature deviation. The remaining processor circuit then has access to an additional module for the range curvature correction so that a point target which is plurivalent per se is assigned to only one distance interval.
This additional module is used for the correction of the curvature of the stored distance intervals; that is, for the correction of the range curvature deviation. If the path error is larger than a specified threshold, preferably larger than half the required radial resolution, this additional module will then assign the received signal proportion to one or more adjacent distance intervals so that it will be correlated with the reference function which is valid there.
During the system design of the ROSAR, the maximal angle of rotation .alpha..sub.RCmax up to which the illumination of a target for a specified radial resolution .DELTA.R.sub.Smin may take place without the requirement of a correction must always be examined. A maximal path difference .DELTA.R.sub.Smax caused by the curvature, for example, half the length of the distance interval .DELTA.R'.sub.Smin is permitted. That is, a comparative value is considered as the maximal permitted path difference in the radial direction .DELTA.R'.sub.Smin. This value may amount to, for example, half the radial resolution .DELTA.R.sub.Smin.
FIG. 9 illustrates the illumination geometry of the curved resolution cells with the circular-ring-shaped distance intervals of the length .DELTA.R.sub.Smin. The target is situated at a distance on the ground R.sub.gn from the pivot and at a distance R.sub.SO from the antenna at the angle of rotation .alpha.=0.degree.. This is so that, for other angles of rotation .alpha.=.omega..sub.o t, deviations .DELTA.R.sub.S =(R.sub.s -R.sub.SO) occur with respect to the curved synthetic aperture S.
The following will therefore apply: .DELTA.R.sub.Smax .ltoreq..DELTA.R'.sub.Smin
with ##EQU1## As long as the slant-distance difference .DELTA.R.sub.Smax occurring at the edges of the synthetic aperture S is smaller than or equal to the specified comparative value .DELTA.R'.sub.Smin, no correction must be made. However, when the maximal path difference .DELTA.R.sub.Smax exceeds the comparative value .DELTA.R'.sub.Smin, a correction must take place.
The presetting of the device with half the radial resolution is not fixedly defined but may, depending on the required precision, also contain smaller limits. For this reason, an adapting factor F is generally introduced. The following will then be valid for the comparative value: ##EQU2## This adapting factor F may, for example, be between 2 and 5.
The following applies to the maximal distance difference .DELTA.R.sub.Smax : ##EQU3## with the angle of rotation range .alpha..sub.s : ##EQU4## wherein n is the number of the respective distance interval.
As long as the condition .DELTA.R.sub.Smax .ltoreq..DELTA.R'.sub.Smin is met, no correction has to be made.
A good approximation for the maximal path difference .DELTA.R.sub.Smax caused by the curvature is obtained for the instance where the distance R.sub.gn is large with respect to the rotor blade length L and the height above the ground H.sub.0, at: ##EQU5## with .alpha..sub.s =.gamma..
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.